This is what I do. I also do this when I'm chosing fabric combinations for a quilt. You know what they say "a picture is worth a thousand words" of course when you are quilting and then have to use the seam ripper because you put it together wrong the picture would save you a thousand words that can't be said in public or at least shouldn't be said in public.

Using my cell phone camera and lots of labels works for me, too. It is so frustrating to get them out of order.

I have a small sewing area and small house without a wall for even a wall to hang a design wall. I lay out a flannel sheet on a table then lay out the blocks. Since I rarely have time to finish the assembly I have taken pictures which I really like. Then I fold up the sheet over the blocks carefully. I can unfold as I am ready to work on the top.

I do both suggestions, pinning to keep in order and marking with sticky notes.

I like your flannel sheet idea, Newby. If you fold carefully or roll it up you could unfold a row at a time as you sew.

This also works with a flannel backed tablecloth. I also take a picture just in case blocks/pieces get mixed up.

I use a design wall.

I have wooden clothes pins. I mark each one by row and number,apply to the blocks in order,then it's easier to keep them where they should go. I'm famous for sewing stuff upside down and backwards...repeatedly!

I use the small yard sale type stickers with the A, b, c, going down and numbers going a cross. A-1, A-2 etc. I also take pictures a a rearrange them and finally after I settle on the final layout.

No need for fancy pins - if you have flower head pins you can write on the "flower" part with a sharpie